util: Fix call_function_periodically.

This function incorrectly and misleadingly did an immediate initial
call, despite both of its callers doing immediate calls themselves (in
one case, with a different parameter passed).

This led to unnecessary server load when reloading the app via event
system triggered reloads, since every client would call `/` twice.

(cherry picked from commit 2c56978b02)

web/src/util.ts

@@ -408,9 +408,12 @@ export function call_function_periodically(callback: () => void, delay: number):
     // calling "callback".
     setTimeout(() => {
         call_function_periodically(callback, delay);
-    }, delay);
 
+        // Do the callback after scheduling the next call, so that we
+        // are certain to call it again even if the callback throws an
+        // exception.
         callback();
+    }, delay);
 }
 
 export function get_string_diff(string1: string, string2: string): [number, number, number] {